Loop powered level measurement systems operate on a 4-20 mA current loop, hence the name loop powered. The circuitry for the level measurement system, i.e. the load, is typically designed to operate at less than 4 mA. The current loop provides a terminal voltage in the range 12-30V, but is nominally 24V.
To take a measurement, power is applied to the transducer and the reflected pulses are detected and the distance to the reflective surface is calculated or measured. If more than 4 mA is needed to make a measurement, then energy taken from the current loop is stored until there is enough to make the measurement. In addition, to make rapid measurements, more current from the loop is also needed. As the current in the loop increases, the speed of measurement also increases. Since the power available from the current loop is less than the power required to continuously operate the level measurement device, the level measurement device is operated intermittently. In a typical level measurement system, measurements are taken once every second up to once every five seconds.
Accordingly, there remains a need for power management in the field of loop powered level measurement or time of flight ranging systems.